The field of the invention is fluid catalytic cracking (FCC) and hydrocracking.
FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier, less valuable hydrocarbon feed stocks such as gas oil.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, coke tends to accumulate on the catalyst which is burned off in the regenerator.
The least valuable product from an FCC process is clarified slurry oil (CSO) which is withdrawn from the bottom of the FCC main fractionation column and burned as fuel. The CSO comprises the heaviest product mixed with catalyst particles that have not been successfully removed from the FCC products. Light Cycle Oil (LCO) is also produced in an FCC unit and can be directed to the diesel pool. However, LCO may degrade the quality of the diesel pool due to its high aromaticity and low cetane value. The CSO is less valuable than Light Cycle Oil. Due to operational constraints of FCC main fractionation column, the CSO leaves the main fractionator with an appreciable amount of hydrocarbons in the boiling range of LCO and a small amount in the boiling range of gasoline. Heavy cycle oil (HCO) is an FCC product pumped around to cool the main fractionation column but is not often recovered from the main fractionation column.
For FCC units experiencing coking in the main fractionation column bottoms, the main column can be operated at a lower temperature by reducing the flow rate of LCO withdrawn from an LCO side outlet of the main fractionation column and increasing the flow rate of CSO withdrawn from the main fractionation column bottom. The additional LCO in CSO will lower FCC main fractionation column temperature, reduce coking and maintenance on FCC main column bottoms exchangers. However, the refiner who operates with extra LCO in CSO bottoms stream to prevent maintenance issues pays a penalty by losing the additional LCO in the CSO which is not recovered.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack at the carbon-carbon bonds in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more fixed beds of the same or different catalyst. Hydrotreating is a hydroprocessing process in which heteroatoms are removed from hydrocarbons and olefinic compounds are saturated.
It would be desirable to recover useful hydrocarbons from CSO. It may also be desirable to upgrade the hydrocarbons recovered from CSO to make quality diesel.